(2) and (3) are the same, so they could be interchanged at any time. Both of them could be tranlasted as I 've had dinner. While the (1) is a little complicated. It might by used as a future tense, like I'm going to eat, can I? In such situtaion, you could use a rising intonation. But when it is put in a falling intonation, it could mean the same as (2) and (3).

I remember getting really confused as well. The problem is that is very difficult to explain these sentences without context. I think it is just a case of listening to as much Chinese as possible. Where in English I think you can take these verb structures more out of context and describe their meaning well, it's more difficult to do so in Chinese. for instance 吃了饭 for instance 我吃了饭，再。。。。 Once I've eaten, I will 我吃了饭，又去了。。。 Once I had eaten, I went to 昨天我吃了饭 I ate yesterday You kind of need to know the whole background story with a scenario to properly translate it. Often it can get confusing because we use tone (rising, falling etc) and stress in English to describe things feelings, scenario, which might be described using a sentence pattern in Chinese. So it can get confusing cos of lack of context and perhaps someone explaining doesn't get a subtle difference. I know that doesn't answer your question really, but just a pointer as to why it can be so frustrating.

1- V了 ---this V（activity） has done or other situations have come out 2- Sentence + 了 （“了” at the end of sentence ） ---- to emphasize and certain the content of this sentence or phrase， to let sb notice this content. This content may has different connections with your states. 3- V过--- sb has an experience of this V（activity）， and this activity is not doing right now. "我吃饭了” =point 2 -- A- you may want others to notice that you are ready to eat now "我吃饭了！” B- you may want others to notice that you are not hungry now, because you've eaten “我已经吃饭了” C- you may want to make it as a condition then state other content “我吃饭了再去找你” “我吃了饭了”= point 1 and 2--- the first “了” means this activity has completely done. ---- the second "了” means you want others to notice all this sentence's information, or make it as a condition then state other content "我吃过饭了”= point 3 and 2-- “过” means you have an experience and right now this activity has stopped "了” ----pls notice the statement above

1) there doesn't seem to be such an expression in real life because the concept of time in this sentence is unclear. Nobody would understand what you mean exactly with it. 2)"I have eaten lunch/dinner". With two "了", one after the verb and the other at the end of the sentence, it expresses your current state while explaining something is finished, hence simple present perfect. 3) the same meaning as 2). "过" can be used to replace the first "了" after the verb.

The first one is past tense, second emphasizes past tense, the third one is same with the first,but sometimes,我吃饭了means 我要吃饭了 in specific intonation